Feline Leukemia Virus

Feline leukemia is a contagious, untreatable disease that can be fatal.

Feline leukemia has been linked to the development of certain cancers in cats.

Cats that go outside are at increased risk for exposure to feline leukemia.

Vaccination can aid in the prevention of disease associated with feline leukemia.

What Is Feline Leukemia?

Feline leukemia virus (FeLV) is contagious among cats. Unlike many other viruses that enter specific cells in the body and destroy them, FeLV enters certain cells in a cat’s body and changes the cells’ genetic characteristics. This permits FeLV to continue reproducing within the cat each time infected cells divide. This allows FeLV to become dormant (inactive) in some cats, making disease transmission and prognosis (outlook) difficult to predict.

How Do Cats Become Infected With Feline Leukemia?

Feline leukemia is generally transmitted through contact with saliva from an infected cat. Certain “social” behaviors such as mutual grooming and sharing food or water bowls can spread the disease. Kittens can become infected during fetal development or during the first days of life as their mothers nurse and care for them.

FeLV is killed by many disinfectants and does not live for very long in the environment, so contact with an infected cat is necessary for disease spread. However, predicting which cats can transmit the disease is complicated because some cats that are contagious don’t develop signs of infection.

Signs of Feline Leukemia

Not every cat that becomes infected with FeLV develops clinical signs or long-term complications associated with the virus. The immune system of some cats can eliminate the infection before the cat becomes sick. In other cats, the virus can “hide” in the bone marrow, where it is difficult to detect until it begins to cause problems later in life. Still other cats become carriers of the disease or experience various illnesses and immune suppression before eventually dying of FeLV-associated complications.

Because FeLV can affect almost any organ system in the body, clinical signs can vary significantly. Some of the signs are:

Anemia (lack of red blood cells)

Leukemia

Immune suppression

Fever

Lethargy (tiredness)

Chronic respiratory infections

Chronic dental, oral, and gum infections

Cancer of the lymphatic system (and other cancers)

Diagnosis and Treatment

Because there are several stages of disease and not every cat handles FeLV infection the same way, diagnosis is not always straightforward. Blood tests detect the disease in many cats, but for other cats, the bone marrow must be examined to confirm infection. Some cats may test positive on blood tests when they are young kittens but test negative later on if their immune system has been able to eliminate the infection. Similarly, some cats may test negative at one point and test positive later on, as the virus progresses through various stages in the body. Because FeLV infection can have many clinical presentations, your veterinarian may want to test your cat if he or she seems to be ill—especially if a fever is present. Some cats need to have multiple tests done to confirm infection.

No medication can eliminate FeLV. Most treatments are aimed at managing the clinical signs and complications. Cats that are anemic may receive blood transfusions; cats that are dehydrated or not eating may receive intravenous fluids and feedings; chemotherapy is sometimes helpful in managing cancers; and antibiotics may be used to treat associated infections.

Vaccination and Prevention

Several vaccines are available for preventing disease associated with FeLV. All of the available FeLV vaccines have been tested and found to be safe and effective when administered as directed.

Kittens are generally vaccinated against FeLV around 8 to 9 weeks of age. A booster vaccination is given 3 to 4 weeks later according to the vaccine label, followed by boosters each year as long as the risk for exposure remains. Cats that go outside or live with other cats are at greater risk for exposure to FeLV compared with cats that stay indoors and have limited contact with other cats. If risk for exposure is low, your veterinarian may not recommend the FeLV vaccine for your cat. Ask your veterinarian about how to protect your cat from this disease.

Because FeLV is transmitted through contact, keeping sick cats separated from healthy cats can reduce the likelihood of transmission. Any new kitten or cat being introduced into the home should be examined by a veterinarian as soon as possible and separated from all other household pets for a quarantine period of at least a few weeks. During that time, the new cat should be tested for FeLV and monitored closely for any signs of illness. Any problems should be reported to your veterinarian before introducing the new cat to your other pets.